Non-aqueous electrolyte secondary batteries used as power sources for mobile communication devices and portable electronic devices in recent years are characterized by high potential force and high energy density. Examples of positive electrode active materials used for non-aqueous electrolyte secondary batteries include lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), manganese spinel (LiMn2O4), etc. These active materials have a voltage of not less than 4 V relative to that of lithium. On the other hand, a carbon material is usually used in the negative electrode, which is combined with the above-mentioned positive electrode active material to give a 4V level lithium ion battery.
The need has been increasing for batteries not only with high energy density, but also with improved high rate characteristics and improved pulse characteristics. Charging/discharging at a high rate imposes an increased load on the active material, making it difficult to maintain the cycle life by conventional techniques.
Some devices require batteries that have such high rate discharge performance and yet exhibit a flat battery voltage in the charge/discharge curves. Batteries with a positive electrode active material having a layered structure such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2) usually exhibit relatively flat S-shaped charge/discharge curves. Accordingly, it is difficult to maintain a flat charge/discharge voltage during high rate charging/discharging. Since the positive electrode active material repeatedly expands and contracts to a great degree in the layer direction during charging/discharging, the stress resulting therefrom reduces the cycle life particularly at the time of high rate charging/discharging.
The positive electrode active materials are recognized to have relatively flat-shaped charge/discharge curves. However, from the viewpoint of determination of the remaining capacity, they are considered as not suitable for determining the remaining capacity because accurate analysis in a narrow potential range is necessary. Particularly, when lithium is intercalated into the negative electrode during charging, the potential of the negative electrode rapidly drops to about 0.1 V and, after that, the negative electrode absorbs lithium at a given potential. As for the positive electrode active materials, since LiMn2O4 having a spinel structure in particular exhibits flatter charge and discharge curves than lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) both having a layered structure, LiMn2O4 is considered as not suitable for determining the remaining capacity.
In order to determine the remaining capacity of a non-aqueous electrolyte secondary battery, usually, current and time other than voltage are detected and a calculation is then made in an integrated circuit based on the above information to yield the remaining capacity of a battery, which is typified by, for example, the method of Japanese Laid-Open Patent Publication No. Hei 11-072544.
In order to monitor the completion of charging, Japanese Laid-Open Patent Publication No. 2000-348725 proposes to use LiMn2O4 as a positive electrode active material and Li4Ti5O12 and natural graphite as negative electrode active materials. This technique enables the monitoring of the completion of charging by creating a potential difference in the potential of the negative electrode. The reference discloses a negative electrode comprising Li4Ti5O12 whose potential persists at 1.5 V and natural graphite whose potential persists at 0.1 V.
Now, the following describes a conventionally proposed battery system comprising a positive electrode containing a conventional positive electrode active material having a spinel structure and a negative electrode containing a lithium-containing titanium oxide having a spinel skeleton.
For example, Japanese Laid-Open Patent Publication No. Hei 11-321951 proposes a positive electrode active material represented by the formula: Li(1+x)Mn(2−x−y)MyOz, where 0≦x≦0.2, 0.2≦y≦0.6, 3.94≦z≦4.06, and M is nickel or a compound composed of nickel as an essential component and at least one selected from aluminum and transition elements, and a method for synthesizing the positive electrode active material without an impurity of NiO. To be specific, a mixture comprising a manganese compound and a metal M compound is baked at 900 to 1100° C., and the mixture is baked again with a lithium compound.
This method, however, involves a reaction between manganese and a metal M, that is, a reaction between solids. Accordingly, it is difficult that the above two is incorporated uniformly. In addition, since the baking is performed at a high temperature of not less than 900° C., reactivity with lithium is reduced after the baking, making it difficult to obtain the desired positive electrode active material.
Japanese Laid-Open Patent Publication No. Hei 9-147867 discloses a positive electrode active material comprising an intercalation compound having a spinel crystal structure and being represented by the general formula: Lix+yMzMn2−y−zO4, where M represents a transition metal, 0≦x≦1, 0≦y<0.33, and 0<z<1. The disclosed positive electrode active material is capable of charging/discharging at a potential of not less than 4.5 V relative to that of Li/Li+.
Japanese Laid-Open Patent Publication No. Hei 7-320784 discloses a battery comprising a positive electrode containing Li2MnO3 or LiMnO2 as an active material and a negative electrode containing lithium-intercalated Li4/3Ti5/3O4 or LiTi2O4 as an active material. Japanese Laid-Open Patent Publication No. Hei 7-335261 discloses a battery comprising a positive electrode containing a lithium cobalt oxide (LiCoO2) and a negative electrode containing a lithium titanium oxide (Li4/3Ti5/3O4). Further, Japanese Laid-Open Patent Publication No. Hei 10-27609 discloses a battery comprising: a negative electrode containing, as an active material, lithium, a lithium metal, or a lithium-titanium oxide with a spinel-type structure; a positive electrode containing, as an active material, a lithium-manganese oxide with a spinel-type structure (Li4/3Mn5/3O4); and an electrolyte comprising a solvent mixture of not less than two components such as LiN(CF3SO2)2 and ethylene carbonate.
Japanese Laid-Open Patent Publication No. Hei 10-27626 discloses to use a lithium-containing transition metal oxide (LiAxB1−xO2) as a positive electrode active material and a lithium-titanium oxide (Li4/3Ti5/3O4) as a negative electrode active material, and to set the actual content ratio of the negative electrode active material to the positive electrode active material to be not greater than 0.5. Japanese Laid-Open Patent Publication No. Hei 10-27627 discloses to use a lithium-manganese oxide (Li4/3Mn5/3O4) as a positive electrode active material, and a lithium-titanium oxide (Li4/3Ti5/3O4) and lithium in the negative electrode, and to set the molar ratio of the lithium-titanium oxide to the lithium-manganese oxide to be not greater than 1.0, and the molar ratio of the lithium to the lithium-titanium oxide to be not greater than 1.5.
Furthermore, Japanese Laid-Open Patent Publication No. 2001-243952 discloses a lithium secondary battery comprising: a positive electrode containing a positive electrode active material represented by the formula: Li1−xAxNi1−yMyO2, where A is one or more selected from alkali metals except Li and alkali earth metals, M is one or more selected from Co, Mn, Al, Cr, Fe, V, Ti and Ga, 0≦x≦0.2, and 0.05≦y≦0.5, and comprising secondary particles formed by the aggregation of primary particles with a mean particle size of not less than 0.5 μm; and a negative electrode containing, as a negative electrode active material, a lithium-titanium composite oxide represented by the formula: LiaTibO4, where 0.5≦a≦3, and 1≦b≦2.5.
Still furthermore, Japanese Laid-Open Patent Publication No. 2001-210324 discloses a battery comprising: a positive electrode containing, as a positive electrode active material, a lithium-manganese composite oxide represented by the composition formula: Li1+xMyMn2−x−yO4−z, where M is one or more selected from Ti, V, Cr, Fe, Co Ni, Zn, Cu, W, Mg and Al, 0≦x≦0.2, 0≦y<0.5, and 0≦z<0.2, having a half peak width of the (400) peak of not less than 0.02 θ and not greater than 0.1 θ (θ is an angle of diffraction) obtained from a powdered X-ray diffraction using CuKα radiation, and whose primary particles are octahedron in shape; and a negative electrode containing, as a negative electrode active material, a lithium-titanium composite oxide represented by the composition formula: LiaTibO4, where 0.5≦a≦3.1, and 1≦b≦2.5.
Some of the conventional techniques, however, cannot completely solve the above-mentioned problems such as improving high rate characteristics and pulse characteristics. For example, charging/discharging at a high rate imposes an increased load on the active material to cause structural damage, thus making it difficult to maintain the cycle life. In addition, since a lithium cobalt oxide and a graphite material, both having a layered structure, repeatedly expand and contract to a great degree in the layer direction during charging/discharging, a stress is given to the active material and an electrolyte exudes from between electrodes, thus reducing the cycle life particularly at the time of high rate charging/discharging. Accordingly, in order to extend the cycle life of such batteries, it is important to prevent the expansion and contraction of the active material.
Batteries used as power sources for electronic devices preferably exhibit a flat-shaped discharge curve, and are required to exhibit a flat voltage even during such high rate discharging. However, batteries currently in practical use exhibit either an S-shaped discharge curve in which the voltage gradually decreases, or a flat discharge curve in which the battery voltage suddenly decreases at the end of charging. The former has the problem that it should have a flatter voltage although it is not difficult to monitor the remaining capacity thereof. In the case of the latter, on the other hand, the voltage difference is extremely small until the end of discharging, so that it is very difficult to monitor the remaining capacity of the battery. Accordingly, obtaining a battery whose remaining capacity can be moderately monitored remains one of the problems.
In view of the above, an object of the present invention is to solve these problems. To be specific, an object of the present invention is to provide a non-aqueous electrolyte secondary battery with improved rate characteristics, improved cycle life, improved safety and improved storage life designed by optimizing the composition and crystal structure of a positive electrode active material, a method for synthesizing the above, the selection of battery systems, an electrolyte, current collector materials for positive and negative electrodes, a separator, the content ratio between positive and negative electrode active materials, and the like. The present invention further provides a non-aqueous electrolyte secondary battery containing a positive electrode active material having flat charge/discharge curves and whose remaining capacity can be easily monitored by deliberately creating a voltage difference at the end of discharging.